1. Field of the Invention
The present invention relates to an optical amplifier and a method for controlling the optical amplifier, and more particularly to an optical amplifier provided with a function of suppressing a light surge caused by a light interruption in an optical relay transmission system or the like, and also a method for controlling the latter optical amplifier.
2. Description of the Related Art
There are some cases in an optical amplifier where a light surge occurs if signal light to be inputted is interrupted. In the conventional optical amplifier, in order to suppress the light surge caused by the interruption of the input light, a part of the input light is branched off by an optical splitter so that a part of the output light is detected by an output light monitor. The condition of the output light is supervised, and a pumping light source driver circuit is controlled by an output controller in the case where the interruption of the input light is detected. The output of the pumping light source is thus controlled.
A part of the input light is branched off by the splitter and detected by the input light monitor. The input photoelectric power level is compared with a predetermined photoelectric level at the input light reduction detector. When the input photoelectric power level is less than the predetermined photoelectric power level, the input light reduction detector feeds a signal to the output controller. A control target of the output constant control is lowered in accordance with this signal to thereby change the operational mode to the output reduction condition. By these operations, it is possible to suppress the light surge even if the input is abruptly raised because the optical amplifier is brought into a low gain condition when the input light is less than the predetermined photoelectric power level.
With such an arrangement, it is possible to suppress the light surge in the case where the input light is abruptly raised a sufficient time after the input photoelectric power has been lowered for changing the mode to the output reduction condition when the input photoelectric power is less than the predetermined photoelectric power level.
However, with the above-described arrangement and method, in the case where the input light is abruptly raised in a short period of time in the range of about several milliseconds to several tens of milliseconds or less after the input photoelectric power has been lowered, there is a problem that the optical amplifier has a gain and the light surge occurs. Accordingly, it is impossible to completely suppress the light surge in accordance with a condition of the signal light to be inputted.